Ametropia refers to significant refractive errors of the eye. Light is focused onto the retina mainly through a combination of refraction by the cornea-air interface, and through the ocular lens. Accommodation (adjusting the eye's focus) occurs through changes in the lens, which fine-tune the refractive power of the eye. Approximately 42 diopters (dpt) of the total refractive power of about 60 dpt of the eye is due to the curvature of the cornea, which does not change during accommodation. Ametropia includes myopia (near-sightedness), hyperopia (farsightedness), and astigmatism (cylindrical and other aberrations). All forms of ametropia can be corrected at least partially, by glasses or contact lenses.
Surgical corrections vary with the kind of ametropia. Most people with ametropia have myopia, which is surgically corrected by reducing the curvature of the cornea. Laser-Assisted in situ Keratomileusis (LASIK), photorefractive keratotomy (PRK), and similar techniques remove a thin lenticular layer of tissue from within the stroma of the central cornea, reducing its outer curvature. LASIK is by far the most frequently used technique. A laminar cut is made in the stroma using a microtome or a short pulsed laser, which creates a thin flap that remains attached at one edge. When a laser is used to create the flap, cutting is carried out by a scanning laser focused within the corneal stroma, followed by a ring-shaped cut through the outer surface of the cornea. The opened flap exposes the cornea stroma. An excimer laser is then used to remove a precise, lenticular-shaped, thin layer of cornea tissue, followed by closing the flap, which re-attaches by adhesion. The pattern and amount of tissue removed is precisely determined to produce the desired reduction in cornea curvature. Astigmatism can also be corrected by LASIK.
Novel, but not yet approved refractive procedures like ReLEx® (Refractive LEnticule Extraction), e.g., femtosecond lamellar extraction (FLEx), and SMILE (SMall Incision Lenticule Extraction) remove stromal lenticule by using femtosecond laser surgery, which is able to make precise cuts within the cornea without damage to the surface of the cornea. Radial keratotomy and similar techniques that injure the cornea in a peripheral pattern, reducing the central cornea curvature after healing, are highly invasive and not predictable enough and therefore not currently employed. For hyperopia correction, LASIK, radial keratotomy and similar techniques either do not work well, or at all.
To correct hyperopia, it is known that the curvature of the surface of the cornea must be increased. A variety of corneal rings have been devised to treat hyperopia. The early corneal rings were made from sutures, which acted like a purse string around the central cornea. An annular cut is made into the cornea around its central axis, and the ring is embedded into the stroma. By tightening or otherwise adjusting the ring, tension is partially relieved on the central cornea, causing its curvature to increase. Unfortunately, corneal rings are problematic compared with LASIK. A substantial injury is produced, the ring typically requires multiple adjustments, tension is concentrated at the ring itself, the degree of correction is difficult to predict, and it is difficult to correct astigmatism. Corneal rings pose so many problems that most patients with hyperopia elect not to use them.